The present invention relates to an interferometer for X-rays, in particular hard X-rays, for obtaining quantitative phase contrast images and measuring wavefront shapes.
For hard X-rays, the cross section for absorption, which generates the contrast in conventional radiography, is usually much smaller than that for elastic scattering. The elastic scattering causes a phase shift of the wave passing through matter. If, for example, 17.5-keV x-rays (which are commonly applied for mammography) pass through a 50-μm-thick sheet of biological tissue, the attenuation will only be a fraction of a percent, while the phase shift is close to π. Thus, the possibility to record the phase shift of x-rays opens the potential for greatly enhanced contrast and, in consequence, reduction of the applied x-ray dose. Reduction of the dose is desirable i) because of health risks for patients exposed to x-rays, and ii) because of the reduced exposure times.
Several methods to detect phase variations in the radiation behind the sample were developed in the past years. They can be classified into interferometric methods, techniques using an analyzer crystal, and free-space propagation methods. These methods differ in the nature of the signal recorded, the experimental setup, and the requirements on the illuminating radiation (especially its spatial coherence and monochromaticity). Although some of them yield excellent results for specific problems, none is very widely used. In particular, none of them has so far found medical diagnostics applications, which require a large field of view of many centimeters, the efficient use of broadband radiation as provided by laboratory x-ray generators and a reasonably compact setup. In addition to medical applications, any investigation of low contrast objects, e.g., in biology or materials science could benefit from exploiting phase contrast. It should be mentioned that an object embedded in a matrix of the same absorption coefficient is completely invisible in absorption contrast, while for the same sample significant differences in phase shift may occur.
The use of gratings as optical elements in hard X-ray phase imaging has the potential of overcoming the problems that so far impair the wider use of phase contrast in X-ray radiography and tomography. Several different geometries of grating-based interferometers for hard x-rays have been investigated recently. The following contains a list of topics and results that have already been published, patented or made available in the past:
The international patent application WO 2004/071298 A1 describes the use of three gratings (two phase gratings and one amplitude grating) to obtain phase contrast images using a polychromatic, incoherent x-ray source. Further publications contain a description of results obtained with a grating based interferometer using two phase gratings and a Bragg analyzer crystal, or a phase grating together with an amplitude grating.
The experimental results known in the prior art were obtained at synchrotron x-ray sources, which are highly expensive installations and are only available at distinct scientific facilities.
At optical or x-ray wavelengths, the phase of a wave cannot be measured directly, so any conversion of a phase shift into an intensity modulation has to depend on interference of two (or more) waves. In order to be able to interfere constructively or destructively, the waves need to have a well-defined phase relation in time and space, i.e. sufficient temporal (longitudinal) coherence and spatial (transverse) coherence.
The commercial impact of an invention in context with radiography will greatly depend on whether an x-ray tube is suitable as radiation source or whether the method is restricted to use at synchrotron radiation facilities because of the required degree of coherence. Thus, a thorough understanding of the relevant terms and relations is essential to appreciate the advantages of the interferometer set-up according to the present invention.
Temporal coherence is related to the monochromaticity of the radiation. For radiation of a bandwidth of δλ around a central wavelength λ, the longitudinal coherence length is λ2/δλ. When considering two beams originating from the same source point that are superimposed after taking different paths through an optical set-up, these beams only have a well-defined phase relation if the difference in optical path lengths is shorter than the longitudinal coherence length. While for visible laser light λ2/δλ can extend over many meters, it is only in the order of a micron at hard x-ray wavelengths even when a crystal monochromator (λ/δλ≈104) is used.
Spatial coherence is related to the size and distance of the source. When considering an intrinsically incoherent and chaotic source (e.g. a light bulb or a conventional x-ray tube) of transverse size c emitting at a wavelength λ, then, at a distance l from the source, the wave-front phase difference between two points lying in a plane normal to the optical axis and separated by a distance r is well defined only if the condition r<λ·1/c is fulfilled. t=λ·1/c is called the transverse coherence length. Most importantly, interference effects such as those used in the grating-based interferometers can only occur when the coherence length is approximately equal to or larger than the relevant length scale of the diffraction aperture or phase mask. For an x-ray tube source with a spot size of 0.1 mm emitting at λ=0.1 nm, the transverse coherence length at a distance of 1 m from the source is again in the order of only one micron.